1. Field of the Invention
The present invention relates to an image forming apparatus that employs an electrophotographic recording process such as a laser printer, a copying machine, and a facsimile.
2. Description of the Related Art
Conventionally, a phenomenon which is called a white gap has been known in color image forming apparatuses, in which a white gap which should not be originally present appears between images formed adjacent to each other in different colors. The phenomenon occurs resulting from the fact that an electrostatic latent image in which drum surface potential steeply changes, for example, an image edge portion, is formed on a photosensitive drum, and when this region is developed by a development device, a visualized image is formed thinner than original. For example, in an image in which a cyan color band and a black color band are formed adjacent to each other, essentially the cyan color band and the black color band ought to be adjacent to each other, but respective visualized images are formed thin, and a gap is produced between cyan color and black color in a final image on a recording material.
FIG. 11 illustrates the detail of a white gap related to the conventional technique and a state of an electric field between a development roller and a photosensitive drum. Thinning of a visualized image of visualized image portion which is a cause of the white gap is attributed to winding up of an electric field at an edge portion of an electrostatic latent image of an electrostatic portion formed on the photosensitive drum.
With respect to this issue, there is known a method for preventing an image from thinning by causing a light emitting element of a laser scanner to emit weak light to an extent not to attach the toner to a non-image portion (non-toner image forming portion) in the whole surface of printable region. Hereinbelow, the method is referred to as background exposure, or non-image portion weak light emission.
The purpose to perform non-image portion weak light emission is not limited to prevention of the white gap. As discussed in Japanese Patent Application Laid-Open No. 2003-312050, the non-image portion weak light emission is also executed as measures to decrease a transfer potential contrast and to prevent an image distortion associated with an aerial discharge which is generated at transfer a nip portion. In other words, the non-image portion weak light emission is not limited to specific purpose.
As a concrete approach for the non-image portion weak light emission, for example, a method called a pulse width modulation (PWM) process for changing a duty ratio of a pulse wave is discussed in Japanese Patent Application Laid-Open No. 2003-312050. The method is to cause a light emitting element of a laser scanner to emit light in a non-image portion at a pulse width equivalent to a weak light emission amount in synchronization with an image clock with a fixed frequency.
In recent years, further higher image quality has been requested for color image forming apparatuses. Under such situation, in addition to an adjustment of a light emission amount corresponding to an image portion, appropriate adjustment of a light amount in the weak light emission of the non-image portion described above is an important issue.